Low density cement slurry and its use

ABSTRACT

A 9 to 12 ppg (1.08 to 1.4 kg/liter) cement slurry for use in oil well type completions which comprises hydraulic cement about 10 to about 30 weight percent hollow glass microspheres based on the weight of the cement and sufficient water to form a pumpable slurry with an API free water content of no more than about 2 volume percent. This slurry is preferably mixed with an amount of water required to produce a slurry with the hydraulic cement having at least an API minimum water content and an API free water content of no more than about 2 volume percent and an additional amount of water equal to about 1.3 to about 1.8 weight percent water based on the weight of the hydraulic cement for each weight percent of the microspheres. The microspheres have true particle densities of about 0.2 to about 0.5 gm/cm 3 , hydrostatic collapse strengths of at least 500 psi (3447 kPa) and average particle diameters of less than about 500 microns.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation-In-Part of United States PatentApplication Ser. No. 932,052 filed Aug. 8, 1978, now abandoned and, bythis reference, all of the subject matter of such United StatesApplication Ser. No. 932,052 is incorporated herein.

SUMMARY OF THE INVENTION

In oil well type completions of casing strings, weak formations areencountered which necessitate the use of lightweight cement. Thelightweight cement of this invention comprises hydraulic cement, hollowglass microspheres and sufficient water to form a pumpable slurry withan API free water content of no more than about 2 volume percent. Thesemicrospheres have true particle densities of about 0.2 to about 0.5gm/cm³ as determined by ANSI/ASTM D 2840-69, hydrostatic collapsestrengths of at least about 500 psi (3447 kPa) as determined byANSI/ASTM D 3102-72. and average particle diameters of less than about500 microns and can be used at about 8 to about 50 weight percent basedon the weight of the hydraulic cement to satisfactorily produce slurrieshaving densities of less than about 12 ppg (1.44 kg/liter). Thelightweight cements of this invention have lower densities and attainhigher strengths than the previously used cementing compositions withwater added to reduce the density of the compositions and materials suchas bentonite, diatomaceous earth, or sodium metasilicate added to keepthe compositions from separating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a comparison of the strength of the cement of this inventionto the strength of previously used cements.

FIG. 2 shows the strength of hollow glass microspheres used in thisinvention.

FIG. 3 shows the increase in slurry density of a slurry of thisinvention as hydrostatic pressure is increased.

FIG. 4 shows the water required to form the slurries of this invention.

FIG. 5 shows the slurry consistency of the low density cement of thisinvention.

FIG. 6 shows the slurry densities attained by adding hollow glassspheres.

FIG. 7 shows API thickening times of slurries of this invention.

FIG. 8 shows API compressive strengths of slurries of this invention.

DETAILED DESCRIPTION

In oil well type cementing, it has now been found that a low densitycement which comprises hydraulic cement, hollow glass microspheres andwater is superior to previously described lightweight cements. Lowdensity or lightweight cements are used in the completion of wells whichextend through weak subterranean formations to reduce the hydrostaticpressure exerted by a column of the cement on the weak formations.Examples of such formations are the unconsolidated Late Tertiaryformations encountered in the Gulf Coast Region of the United States,shallow coal seams encountered in Wyoming, Muskeg formations encounteredin Canada, and fractured formations encountered worldwide. Theseformations are encountered when drilling wells for the recovery ofsubterranean resources such as oil, gas, minerals and water and thelightweight cement of this invention is useful in completing thesewells. These completions are referred to herein as oil well typecompletions and include but are not limited to completions where thecement slurry is pumped downwardly through the casing in a well andupwardly into the annulus between the casing and the wall of the well,the cement slurry is placed in the annulus between the casing and thewall of a well by grouting techniques and a plug of the cement slurry isplaced in the well for abandonment or for establishing a whipstock.

It is described in Part 3 of a series of articles on Basic Cementing,Oil and Gas Journal, Volume 75, No. 11, Mar. 14, 1977, that "Basically,lightweight slurries are made by adding more water to lighten themixture and then adding materials which keep the solids fromseparating." Bentonite, diatomaceous earth and sodium metasilicate aredescribed as materials which can be added to keep the solids fromseparating when the water is added for reducing the slurry density. Itis also described that slurry densities as low as 10.8 ppg (1.29kg/liter) can be achieved by adding water. This method of producinglightweight cement slurries has the drawback that the addition of morewater increases the cure time and reduces the strength of the resultinglightweight cement to the extent that they cannot be mixed to densitiesof less than about 10.8 ppg (1.29 kg/liter).

The low density cements of this invention are made by adding hollowglass microspheres and sufficient water to hydraulic cement to form apumpable slurry. The additional water required because of the use of themicrospheres is substantially less than that required when water isadded to reduce the slurry density; therefore, the set cement of thisinvention has a substantially higher strength than previously used lowdensity cements and cure time is reduced. Because of the additionalstrength, the lightweight cement of this invention can be formulated atdensities much lower than the slurry density of 10.8 ppg (1.29 kg/liter)as described in the above referenced publication. This is illustrated inFIG. 1 where the compressive strengths of lightweight cements describedin the above referenced publication are compared with the compressivestrengths of the lightweight cements of this invention.

The comparisons shown in FIG. 1 were made with American PetroleumInstitute (API) Class A cements at the API well simulation test schedulefor a 1,000 foot (305 meter) well as described in "API RecommendedPractice for Testing Oil Well Cements and Cement Additives," AmericanPetroleum Institute, Washington, D.C., API RP-10B, 20th Edition, April1977. These comparisons can be made with respect to the above referencedOil and Gas Journal publication where it is described that mostoperators wait for cement to reach a minimum compressive strength of 500psi (3447 kPa) before resuming operation. It is seen in FIG. 1 that a 9ppg (1.08 kg/liter) cement mixture of this invention will attain acompressive strength of about 500 psi (3447 kPa) in 24 hours as comparedto a 13 ppg (1.56 kg/liter) cement mixture using water to reduce thedensity of the cement mixture and bentonite, diatomaceous earth orsodium metasilicate to keep the solids from separating. It is also seenthat an 11 ppg cement mixture of this invention will attain acompressive strength of about 500 psi (3447 kPa) in 12 hours. Thisillustrates that the use of the lightweight cement of this invention canconsiderably reduce the waiting time while the cement is curing to theminimum strength, waiting on cement time (WOC).

The lightweight cement of this invention is also superior to thelightweight cement described in Biederman, U.S. Pat. No. 3,699,701,Maxson, U.S. Pat. No. 3,722,591, Gebhardt, U.S. Pat. No. 3,782,985, andMessenger U.S. Pat. No. 3,804,058. It is described in column 2 at lines35 through 38 of Biederman that lightweight oil well cements may beformed by the addition of float ash, as aggregate, to existing oil wellcements. Float ash is described in column 1 at lines 13 through 15 ofBiederman as the portion of fly ash that floats on water and has aspecific gravity around 0.7. The use of fly ash floaters is alsodescribed in column 2 at lines 7 through 18 of Gebhardt. It is describedin column 2 at lines 17 through 22 of Maxson that a borehole can belined with an insulating liner formed in situ by hardening in place ahardenable, flowable composition consisting essentially of a hardenable,flowable, adhesive cement and a divided, solid, closed-cell material. Asuitable adhesive cement is described in column 3 at line 18 of Maxsonas hydraulic cement. A suitable divided, solid, closed-cell material isdescribed in column 3, at lines 59 through 63 of Maxson as fly ashfloaters. It is described in column 5, at lines 4 through 8 of Messengerthat pumpable cement slurries can be produced by mixing portland cement,anhydrous sodium metasilicate, water, and hollow sealed spheres made ofceramic or glass. Ceramic and glass spheres are compared in column 5 atlines 30 through 37 of Messenger where it is described that ceramicspheres are preferred in wells having hydrostatic pressures upward toabout 2500 psi (17,237 kPa). The water required for producing thepumpable slurry of Messenger is described in column 5 at lines 38through 55 and in column 6 at lines 1 and 2, where it is described thatadditional water is added for the glass spheres and for the sodiummetasilicate. Advantages achieved by adding this extra water aredescribed in column 2 at lines 11 through 16 of Messenger where it isdescribed that the extra water increases the space between the suspendedhollow sealed glass spheres and thus reduces the breakage of thespheres.

The additional water required by Messenger would have the samedetrimental effects as described with respect to the lightweightslurries produced by adding water to reduce the density of the slurriesand bentonite, diatomaceous earth, or sodium metasilicate to keep thesolids from separating. Sample numbers 4a, 8, 9, 9a, 9b and 9c in TableII of Messenger are examples of Messenger's use of additional wateralong with sodium metasilicate to prevent water separation. Messengersaddition of more water increases the cure time and reduces the strengthof the resulting lightweight cement. Therefore, the lightweight cementof Messenger has longer cure times and is inferior in strength to thecement slurries of the present invention which are free from effectiveamounts of additives such as bentonite, diatomaceous earth and sodiummetasilicate. An effective amount of one of these additives is an amountwhich would prevent the separation of water and particles from theslurry.

The API free water content of slurries of portland cement, IG 101 glassspheres marketed by Emerson & Cuming, Inc., and the amount of waterdescribed by Messenger in column 5 at lines 35 through 38 and column 6at lines 1 and 2, and shown in Table 2, is too high for oil well typecompletions.

The API free water content of a slurry is maintained at no more than 2volume percent and preferably less than about 1 volume percent tominimize water separation after placement of the slurry. Waterseparation in a column of cement can form pockets of free water withinthe cement column or reduce the height of the column of slurry. Pocketsof free water within a cement column can cause corrosion of adjacentcasing. Higher percentages of water separation can form channels throughwhich fluid can migrate past the cement column.

The API free water content of a slurry prepared by mixing API Class Hcement with 20.68 weight percent IG 101 hollow glass spheres and 126weight percent water is about 18.4 volume percent. In addition to thewater separation, solids segregated during this API test. The lowerportion of the 250 milliliter graduated cylinder in which this test wasconducted contained 84 milliliters of solids, the middle portioncontained about 46 milliliters of water and the top portion containedabout 120 milliliters of solids floating on the water. This slurry hasthe same components as sample No. 1 in Table 2 of Messenger. The freewater content of another cement slurry prepared by mixing API Class Hcement with 10.34 weight percent IG 101 hollow glass spheres and 84weight percent water is about 16 volume percent. This slurry alsosuffered about the same severe particle separation as described withrespect to the slurry mixed with 20.68 percent spheres. This slurrycontains the amount of water specified in column 5 at lines 53 through55 for the cement and glass spheres. It is described in Messenger that aslurry should contain 5 gallons of water per 94 pounds of cement and 4.4gallons of water per each 10 weight percent glass spheres. This isequivalent to formulating the slurry with about 44 weight percent waterfor the cement and about 3.9 weight percent water for each weightpercent glass spheres. A slurry mixed with this amount of water wouldcontain about 84 weight percent water when mixed with about 10.34 weightpercent glass spheres and about 124 weight percent water when mixed withabout 20.68 weight percent glass spheres. These weight percents arebased on the weight of the cement.

The ceramic spheres described in Messenger are marketed by Emerson andCuming, Inc. under the trade designation FA-A Eccospheres. These ceramicspheres are known in the industry as fly ash floaters or float ash.

It has been found that hollow ceramic spheres have properties whichrender them undesirable as additives in lightweight cements for oil wellcompletions. A 12.7 ppg (1.52 kg/liter) slurry mixed with API Class Acement, about 20 weight percent hollow ceramic spheres and sufficientwater to form a slurry with an API normal water content lost about 10%of its volume when subjected to pressures of less than about 5000 psi(34,474 kPa) and a 10 ppg (1.2 kg/liter) slurry mixed with API Class Acement, about 67 weight percent hollow ceramic spheres and sufficientwater to form a slurry with an API normal water content lost about 10%of its slurry volume when subjected to pressures of less than about 2000psi. The loss of volume by the 10 ppg (1.2 kg/liter) slurry results in asolid plug that is not pumpable. Slurries of this invention do not losetheir pumpability after loss of 10% of their slurry volume underhydrostatic pressure, additionally the loss of slurry volume of theslurries of this invention is not proportional to the amount of hollowspheres mixed with the slurry.

It has also been found that lightweight cements mixed with these ceramicspheres suffer substantial increases in slurry density and reductions inslurry volume when cured under a hydrostatic pressure of about 500 psi(3447 kPa). A 9.7 ppg (1.16 kg/liter) slurry of API Class A cement,ceramic spheres, and sufficient water to form a slurry with an APInormal water content did not shrink or increase in density when cured atatmospheric temperature and pressure. However, a sample of the sameslurry cured at atmospheric temperature and under a hydrostatic pressureof 500 psi (3447 kPa) lost about 10% of its volume and increased indensity to about 10 ppg (1.2 kg/liter).

The effect of hydrostatic pressure on the volume of a lightweight cementslurry mixed with hollow ceramic spheres detracts from the use of theseslurries in oil well type completions. These detrimental effects areparticularly serious when lightweight cement slurries are mixed withhollow ceramic spheres to produce slurries having densities of less thanabout 10 ppg (1.2 kg/liter). The lightweight cement of this inventiondoes not suffer the substantial detrimental effects as shown in thisapplication for lightweight cement slurries produced with hollow ceramicspheres. It will be described with respect to FIGS. 2 and 3 of thisdisclosure that the effect of hydrostatic pressure on the slurries ofthis invention is proportional to the density of the hollow glassspheres selected.

The hollow glass microspheres used to produce the lightweight cement ofthis invention have average particle diameters of less than about 500microns and can be manufactured by the procedures described in Beck, etal, U.S. Pat. No. 3,365,315 and Veatch, et al, U.S. Pat. No. 3,030,215.In Beck, et al, it is described that hollow glass microspheres are madeby passing particles of glass containing a gas forming material througha current of heated air or a flame. The gas forming materials can beincorporated within the glass particles by the simple step of allowingthe particles, either at room temperature or at higher temperaturesbelow melting, to absorb or adsorb the following materials from theatmosphere surrounding the particles: H₂ O, CO₂, SO₂, F₂, etc. It isdescribed in Veatch, et al, that hollow glass microspheres are made bysubjecting a particulated mixture of siliceous material such as sodiumsilicate, a water-desensitizing agent such as boric acid and a blowingagent such as urea to an elevated temperature for a time necessary tofuse the particles and cause expansion of the particles to hollow glassspheres. High strength glass such as borosilicate glass can be used toproduce hollow microspheres having hydrostatic collapse strengths ofgreater than about 5,000 psi (34474 kPa) as determined by the AmericanSociety for Testing and Materials procedure described in ANSI/ASTM D3102-72.

It is illustrated in FIG. 2 that hollow microspheres with average trueparticle densities of about 0.2 gm/cm³ as determined by the AmericanSociety for Testing and Materials procedure described in ANSI/ASTM D2840-69 generally have ANSI/ASTM hydrostatic collapse strengths of lessthan about 500 psi (3447 kPa) while hollow glass microspheres withANSI/ASTM average true particle densities of about 0.5 gm/cm³ haveANSI/ASTM hydrostatic collapse strengths of greater than about 5,000 psi(34474 kPa).

The hydrostatic collapse strength measurements shown in FIG. 2 were madein accordance with the procedure described in ANSI/ASTM D 3102-72. Thepressure required to collapse about 10 volume percent of the hollowglass microspheres is reported as the hydrostatic collapse strength ofthe microspheres. This is thought to simulate the hydrostatic pressureunder which cement slurries containing these microspheres will besubjected during oil well type cementing operations and also simulatesthe isostatic pressure under which these microspheres will be subjectedduring these operations. These microspheres are generally manufacturedto have average particle diameters of about 10 to about 300 microns.

Many of the uses for the lightweight cements of this invention will bein the completion of wells having depths of about 1,000 feet (305meters) to about 6,000 feet (1,830 meters). It is described in APIRP-10B that wells, for API simulated test conditions, having depths ofabout 1,000 feet (305 meters) will have bottom hole pressures of about1020 psi (7,000 kPa) and that wells having depths of about 6,000 feet(1,830 meters) will have bottom hole pressures of about 3870 psi (26,700kPa). Commercially available hollow glass microspheres with ANSI/ASTMtrue particle densities of about 0.3 gm/cm³ and ANSI/ASTM hydrostaticcollapse strengths of about 1,000 psi (6895 kPa) are satisfactory forcompleting API simulated wells to depths of about 1,000 feet (305meters). Commercially available hollow glass microspheres with ANSI/ASTMtrue particle densities of about 0.4 gm/cm³ and ANSI/ASTM hydrostaticcollapse strengths of about 4,000 psi (27579 kPa) are satisfactory forcompleting API simulated wells to depths of about 6,000 feet (1,830meters). In general, higher density and thus higher strength hollowglass microspheres are desirable for completing deeper wells. Glassspheres having ANSI/ASTM hydrostatic collapse strengths of greater thanabout 4000 psi (27,579 kPa) may be more economical for completing wellsto depths of 10,000 feet (3,050 meters) and deeper.

It has been observed that cement slurries of this invention increase indensity as pressure on the slurry is increased. This is illustrated inFIG. 3 where it is shown that slurry density can be compensated for bymixing the slurry with an amount of hollow glass microspheres which willprovide the appropriate slurry density under the hydrostatic conditionswhich the slurry will be subjected. It is shown that a slurry to besubjected to a hydrostatic pressure of about 2,000 psi (13790 kPa)should initially contain a higher concentration of these microspheresthan a slurry to be subjected to a hydrostatic pressure of about 1,000psi (6895 kPa) and that after being subjected to these maximumhydrostatic pressures, both slurries would contain about the sameconcentration of these microspheres as evidenced by their slurrydensities. It is noted from FIG. 3 that a higher percentage of themicrospheres is lost as the hydrostatic pressure is increased from 1,500to 2,000 psi (10342 to 13790 kPa) than is lost as the hydrostaticpressure is increased from 500 to 1,000 psi (3447 to 6895 kPa) Themicrospheres used for the tests shown in FIG. 3 would be satisfactoryfor completing a well with a bottom hole hydrostatic pressure of about1,000 psi (6895 kPa); however, microspheres with higher collapsestrengths may be more economical for use at greater depths.

The low density cement of the present invention is mixed with sufficientwater to form a pumpable slurry with a free water content of no morethan about 2 volume percent and preferably less than about 1 volumepercent as determined by the procedure described in Section 4"Determination of Water Content of Slurry," API RP-10B. The pumpableslurry of this invention preferably has greater than the minimum watercontent and most preferably has about the normal water content, both asdescribed in this section of API RP-10B. A slurry having a minimum watercontent is described in API RP-10B as having a consistency of about 30Bearden units of slurry consistency (B_(c)) while a slurry having anormal water content is described as having a consistency of about 11B_(c). A slurry with less than an API minimum water content is difficultto pump and a slurry with an API normal water content is considered ashaving an optimum consistency for pumping and a satisfactory free watercontent.

It is illustrated in FIG. 4 that a slurry of this invention formulatedwith API Class A portland cement and about 8 to about 50 weight percenthollow borosilicate glass microspheres based on the weight of the cementcan be mixed with sufficient water to provide a slurry with an APInormal water content. The hollow glass microspheres illustrated in thesetests are B37/2000 microspheres marketed by the Minnesota Mining andManufacturing Company and have average particle diameters of about 20 toabout 130 microns. Other microspheres may have other water requirements.This slurry is mixed with sufficient water to form a pumpable slurrywith the portland cement and an additional amount of water because ofthe microspheres equal to about 1.2 weight percent extra water for eachweight percent of the hollow microspheres when the slurry is mixed withabout 8 weight percent of the microspheres based on the weight of thecement and about 2.2 weight percent extra water for each weight percentof these microspheres when the slurry is mixed with about 50 weightpercent of these microspheres by weight of the cement. At about 10weight percent of these microspheres, the API free water content of aslurry mixed with about 1.3 weight percent extra water for each weightpercent hollow microsphere is about 2.5 milliliters or about 1 volumepercent. At about 30 weight percent of these microspheres the API freewater content of a slurry mixed with about 1.8 weight percent extrawater for each weight percent hollow microsphere is about 1 milliliteror about 0.4 volume percent.

The weight percent water for each weight percent of microspheres asshown in FIG. 4 is in addition to the water required to give a pumpableslurry with the cement. This is illustrated by an API Class A portlandcement mixed with about 10 weight percent hollow glass microsphereshaving ANSI/ASTM average true particle densities of about 0.37 gm/cm³and about 59 weight percent water based on the weight of the cement togive a slurry having a density of about 12 ppg (1.4 kg/liter) and an APInormal water content. About 46 weight percent water based on the weightof the cement is required to give a pumpable slurry with the cement andabout 13 weight percent water based on the weight of the cement isrequired to wet the surface of the glass spheres and to give a pumpableslurry with the mixture of cement and glass spheres. Without theadditional water for each weight percent of microspheres, the slurry ofcement, glass spheres and water may not be pumpable.

The water content of the hydraulic cement slurries for use in producingthe mixture of this invention is given in Section 4 of API RP-10B wherenormal and minimum water contents are specified. The hydraulic cementcan contain any conventional additives needed to meet well conditionsand should contain normal to minimum water contents as required whensuch additives are mixed with the hydraulic cement. Additives which maybe desired are accelerators, loss circulation materials and dispersants.The following Table 1 also appears at Section 4 of API RP-10B andprovides the water required for mixing neat API Classes of cement withwater to produce slurries with normal water contents.

                  TABLE I                                                         ______________________________________                                        CEMENT SLURRY COMPOSITION                                                              2                                                                             Water      3                                                         1        Percent    Water                                                     API Class                                                                              by Weight of                                                                             Gallons     Liters                                        Cement   Cement     per 94 lb Sack                                                                            per 42.6 kg Sack                              ______________________________________                                        A & B    46         5.19        19.6                                          C        56         6.32        23.9                                          D, E, F, & H                                                                           38         4.29        16.2                                          G        44         4.97        18.8                                          J        *          *           *                                             ______________________________________                                         *As recommended by the manufacturer.                                     

It is illustrated in FIG. 5 that about 1.3 weight percent extra waterfor each weight percent B37/2000 microsphere based on the weight of thecement will provide a slurry having an API normal water content at about10 volume percent hollow microspheres based upon the weight of cement toa slurry having an API minimum water content at about 40 weight percenthollow glass microspheres.

It is shown in FIG. 6 that an API Class A cement mixed with glassmicrospheres having ANSI/ASTM average true particle densities within therange of about 0.2 to about 0.5 gm/cm³ and an API normal water contentcan be formulated to produce slurries having densities of less thanabout 12 ppg (1.4 kg/liter). Generally microspheres with ANSI/ASTM trueparticle densities within the range of about 0.3 to about 0.4 gm/cm³will be used to produce slurries having densities of about 9 to about 12ppg (about 1.08 to about 1.4 kg/liter).

The lightweight cement of this invention can be formulated with anyhydraulic cement normally used in oil well type cementing and the hollowglass microspheres can be used in combination with other additives.Portland cements are the basic hydraulic cements now being used for oilwell type cementing and are often mixed with accelerators, retarders,dispersants and loss circulation agents to meet specific wellconditions.

The use of calcium chloride as an accelerator in the low density cementof this invention is illustrated in FIGS. 7 and 8 where it is seen thatthe thickening times of the lightweight cements of this inventiongenerally increase as they are mixed with higher concentrations ofhollow glass microspheres and that their compressive strengths generallydecrease at higher concentrations of microspheres. It is also seen thatcalcium chloride has a greater accelerating effect on the lightweightcements of this invention mixed with lower concentrations of hollowglass microspheres. The calcium chloride is diluted by the additionalwater for each weight percent glass microsphere.

Tests have also been conducted on API Class A cement mixed with about25.5 weight percent hollow glass microspheres having ANSI/ASTM trueparticle densities of about 0.37 gm/cm³ and with dispersant and gypsumhemihydrate. The percent microspheres is based on the weight of thecement. Dispersants are known for reducing the water required in themixing of cements and gypsum hemihydrate is known for producing a cementslurry that will attain a high gel strength when movement of the slurryis reduced or terminated. Cement slurries which gel when movement isreduced are useful for plugging fractures or filling voids which extendfrom a wellbore and for reducing the hydrostatic pressure applied by theslurry to weak subterranean formations after placement of the cementslurry has been completed. The addition of 0.75 weight percentdispersant based on the weight of the cement reduced the extra waterrequired because of the addition of the hollow glass microspheres from1.65 to about 1 weight percent water for each percent of thesemicrospheres based on the weight of the cement. The addition of 7.5weight percent gypsum hemihydrate provided a slurry with a gel strengthof about 1,000 pounds per 100 square feet (50 kg/cm²) after movement ofthe slurry had been suspended for about 12 minutes as compared to a gelstrength of about 50 pounds per 100 square feet (2.5 kg/cm²) for aslurry containing no gypsum hemihydrate.

The low density cement of this invention is also useful in abnormallyhot wells because it is characterized on curing by having quite highcompressive strength at elevated temperatures (above 110° C.). In theunusual case in which the API cement (except Class J) is to be curedunder or later subjected to high temperature conditions (above around110° C.), the initial compressive strength is not maintained but maydecrease rapidly, of the order of 30% or more. The lightweight cement ofthis invention is also useful in steam or hot water injection wells andproducing wells from thermal sources and the like, as well as in wellspenetrating permafrost, where there is a definite need to obtain asatisfactory insulating lining between the fluid and the formationssurrounding the well.

It has been observed that the lightweight cement of this invention has ahigh temperature strength at temperatures above 230° F. (110° C.) whichis at least the same order of magnitude as (and sometime exceeds) thatof the material when cured at a temperature in the order of 90° to 120°F. (32° to 49° C.). This is illustrated with respect to the followingexample of a slurry of the present invention. An API Class A cementslurry in the absence of finely divided hollow spheres would containapproximately 46 weight percent of water. If 25.5 weight percent ofB37/2000 spheres are added (ANSI/ASTM average average true particledensity of 0.37 gm/cm³), an additional 34 weight percent of water, about1.3 weight percent water for each weight percent hollow spheres, for atotal of 80 weight percent water can be incorporated in this particularcase. The percentages are based on the weight of the cement. The densityof the resulting slurry is of the order of 9.5 ppg (1.14 kg/liter). Whenthis slurry is cured at about 300° F. (149° C.) it has a one daystrength of about 1,000 psi (6895 kPa). A sample of this lightweightcement exposed to a temperature of 300° F. (149° C.) for seven (7) dayshas a compressive strength of about 970 psi (6688 kPa) which is aninconsequential reduction. For all practical purposes, one can say thetwo compressive strengths are identical. Ordinarily, an API Class A oilwell cement slurry with no low density additive upon curing would have a1 day strength of the order of 3,000 psi (20684 kPa) at 300° F. (149°C.) temperature, but exposed to this temperature of 300° F. (149° C.)for seven (7) days would regress typically approximately 30%. Alightweight cement produced by adding water to reduce the density of theslurry to about 13 ppg (1.56 kg/liter) and bentonite, diatomaceous earthor sodium metasilicate to keep the solids from separating would losesubstantially all of its compressive strength after being cured at 300°F. (149° C.) for one week.

Examples of the hollow glass microspheres which have been found to beuseful in formulating the lightweight cement of this invention are shownin Table II.

                  TABLE II                                                        ______________________________________                                        Free-Flowing Hollow Glass Sphere Properties-Typical                                                        B23/- B37/- B38/-                                Type        1G101   1GD101   500   2000  4000                                 Made by*    E-C     E-C      3M    3M    3M                                   Glass**     SB      SB       SLB   SLB   SLB                                  ______________________________________                                        ANST/ASTM                                                                     average                                                                       true particle                                                                 density, (gm/cm.sup.3)                                                                    0.31    0.3      0.23  0.37  0.38                                 Average particle                                                              diameter,                                                                     (microns)   40-     40-150   20-130                                                                              20-130                                                                              20-130                                           175+                                                              ANSI/ASTM                                                                     hydrostatic collapse                                                          strength at 10 vol.                                                           percent collapse,                                                             (psi)       un-     un-      500    2000  4000                                            known   known                                                     (kPa)       un-     un-      3447  13790 27579                                            known   known                                                     Hydrostatic collapse                                                          strength - volume                                                             percent survivors                                                             at 1500 psi                                                                   (10340 kPa)  47     76.6     un-   un-   un-                                                               known known known                                Softening                                                                     temperature                                                                   (°C.)                                                                              480     480      715    715   715                                 ______________________________________                                         *E-C means Emerson & Cuming, Inc., Canton, Massachusetts 3M means 3M          Manufacturing Co., St. Paul, Minnesota                                        **SB is sodium borosilicate glass, SLB is soda lime borosilicate glass        (Note: in the claims both sodium borosilicate glass and soda lime             borosilicate glass are referred to generically as "sodium borosilicate        glass" or "borosilicate glass." Thermal conductivity is of the order of 8     to 11 (K cal) (cm) (hr) (sq m) (°C.).)                            

The Emerson and Cuming microspheres are thought to be manufactured bythe procedure described in Veatch, et al, U.S. Pat. No. 3,030,215 andthe 3M Manufacturing Company microspheres are thought to be manufacturedby the procedure described in Beck, et al, U.S. Pat. No. 3,365,315. Thecommercially available hollow glass microspheres manufactured by theprocedure described in Beck, et al, have strengths that vary withANSI/ASTM true particle densities as shown in FIG. 2 and are usefulunder the broad range of hydrostatic pressure conditions expected to beencountered in the use of a lightweight cement of this invention.Microspheres manufactured by procedures other than the proceduresdescribed in Beck, et al, U.S. Pat. No. 3,365,315 may not besatisfactory for use under conditions where the lightweight cement ofthis invention is subjected to hydrostatic pressures of greater thanabout 1500 psi (10340 kPa).

One note about use: since these tiny spheres are made of glass, it isapparent that one must be careful in mixing the spheres and cementtogether. We do not find it necessary to use metasilicate; however, wemix by moving these materials into a storage tank by use of somethingresembling a large vacuum cleaner or diaphragm pump, and it is desirablethat the people using these wear respirators and goggles. There shouldbe at least an ordinary vacuum bag filter in the exit line from thevacuum system. At the bulk station, where these mixtures are to be mixedtogether, the spheres and cement are dry mixed in the presence ofsufficient air to cause homogeneous blending of the two materials. Theyare then moved into a truck and sent to the well. At the well anordinary Halliburton jet mixer or equivalent can be employed to mixthese dry ingredients with water to the required density. With the useof pneumatic bulk handling vessels, operators may not be exposed tothese tiny glass spheres and may not need to exercise these safetyprecautions.

The use of a densitometer or a pressurized fluid density balance asdescribed in Appendix B to API RP-10B are about the minimum equipmentcurrently necessary to monitor the density. A manual centrifuge can alsobe used in making the determination of water content. With a manualcentrifuge there is no need to depend upon a source of electric power orthe like. Shortly before the mixing is going to commence at the fieldlocation, we prepare small test samples containing the amount of solidingredients planned for a particular job, each having a calibratedamount of water amounting, for example, to 70%, 80%, 90%, and 100%(based on cement weight). These separate samples are centrifuged whichcauses the material to separate out into a cement portion, a waterportion, and a portion containing finely divided glass spheres. Oneprepares a graph in which the true or calibrating percentage of water isplotted along one axis and that determined from the centrifuge volume isplotted along the other axis. A smooth curve is drawn through thepoints. This calibrates the centrifuge.

Then when the actual mixing of the cement, glass spheres and water takesplace, we take samples every two minutes and centrifuge these in thecalibrated centrifuge. The volume of water found in the sample is readoff against the calibration curve to determine the actual percentconcentration of water in that particular sample. The rapidity of theoperation can be judged by the fact that a typical figure for slurrymixing rate is about 4 barrels per minute (636 liter/min).

To give a specific example of such an operation, 2,000 lbs (907 kg) ofIGD-101 hollow glass microspheres were blended with 85 sacks, 8,000 lbs(3630 kg), of Oklahoma API Class A cement. The cement and the tiny glasshollow spheres were blended in two batches of equal volume. In eachbatch, half the cement was vacuum injected into the blender, then thespheres, and then the remainder of this cement. The batches were blownback and forth from tank to bulk truck three times to mix the hollowspheres with the cement. The blending operation took 11/2 hours,including unpacking the glass spheres. Four cement company personnelwere employed in this operation. Dust was not excessive but goggles anddust masks were worn by all personnel in the blending area. In thisoperation, the glass spheres were dumped into a cone very much like theHalliburton hopper for jet mixing, then were sucked vertically out ofthe cone with a 6 in. (15 cm) diameter vacuum pipe. It was found that aquite homogeneous mixture of finely ground cement (nominally through 300mesh) and the IGD-101 Microballoons was obtained by this procedure.

This material was then mixed to form the lightweight cement slurry.During this, a special Halliburton loop densitometer with a range from8.3 lbs/gal to 10.8 lbs/gal (0.99 to 1.29 kg/liter) was employed, whichhad been calibrated with water at 8.34 ppg (1 kg/liter). The mixing wasdone with a jet mixer and a pump truck equipped with two HalliburtonT-10 pumps, i.e., typical oilfield equipment. The slurry was mixed atdifferent feed water pressures ranging from 125 to 475 psi (8.79 to 33.4kg/cm²). The slurry was best mixed at a feed water pressure of 475 psi(33.4 kg/cm²). Then, as mentioned above, mixed at a rate of 4 barrelsper minute (636 liter/min.), the slurry was quite adequately pumpable,although it appeared somewhat thick. The centrifuge showed the watercontent to be in the range of 80% to 85%, which was the objective; apycnometer showed slurry specific gravity of the range of 1.03 to 1.14.The loop densitometer chart showed ranges of specific gravity from 1.04to 1.06 which upon calibration against the pycnometer measurementsshowed the densitometer to be substantially accurate.

The field processing of the lightweight pumpable hydraulic cement slurryas has been described is straightforward and essentially is as follows:the homogeneously blended dry materials, namely spheres and cement, aremixed at the well with the amount of water equivalent to (a) thecustomary amount of water (API normal to minimum water contents) plus(b) the excess based on the concentration of spheres, as describedabove. This produces a slurry with consistency in the range from amaximum consistency of 30 B_(c) (API) to one with a free water notexceeding 2 volume percent (API) (tests for consistency in Beardens andfor percent free water are as specified in API code RP 10B). When soprepared, the material is pumped by ordinary cementing trucks using theusual cementing procedures. In one such procedure, the cement is pumpeddownwardly through the casing then flows up and around it, permittingcasing manipulation such as scratching, rotating, oscillating, etc., todisplace the drilling mud and do a good cement job.

After the cement placement, we find that ordinary waiting on cement(WOC) times result. The development of strength as the slurry setsdepends not only on temperature and pressure but on cement class, watercontent and presence of additives. Oil well cementing slurries of thisinvention develop relatively high compressive strengths within 12 hoursas illustrated in FIG. 1, whereas previously used low density slurriesdeveloped only low strengths after 24 hours. This is particularly truefor slurry densities in the range of about 1.08 to about 1.4 kg/liter(about 9 to about 12 pounds per gallon).

It is extremely time consuming to determine the reaction of cements tothe presence of all types of cement additives. However, tests to date donot indicate any difference in the use of the cement additives withthese particular slurries, compared to those prepared in the absence ofthe finely divided hollow spheres. In other words, such things asdispersants, accelerators, and the like operate essentially in the samefashion as before.

It should be emphasized that the cured or set cement (the solidresulting from the slurry already described) has an additional valuableproperty in addition to high mechanical strengths and low densities.This is that it has considerably less thermal conductivity than ordinarycements cured from slurries which do not contain these hollow spheres.As is the case for other heat insulators, the presence of the inclosedgas in the very large number of hollow spheres incorporated into the setsolid gives a marked decrease in thermal conductivity. This is importantwhen setting cement against permafrost or where well fluids are quitehot.

Two casing strings have been experimentally completed with thelightweight cement of the present invention. In both completions,circulation of cement to the mudline was observed with a remote camera.These casing strings were the 24 inch (610 mm) and 16 inch (406 mm)casing strings which, respectively, extended 600 and 1,000 feet (183 and305 m) below the mud line in 1,000 feet (305 m) of water. Previousattempts on similar wells to circulate commercially available lowdensity cements to the mud line had failed.

The 24 inch (610 mm) casing string was cemented with about 2,456 cubicfeet (70 m³) of slurry formulated by mixing about 900 sacks (84,600pounds, 38370 kg) of API Class A Portland cement with about 1,300 pounds(590 kg) of calcium chloride, about 144 (65 kg) pounds of defoamer,about 22,500 pounds (10,206 kg) of B37/2000 hollow glass microspheresand sufficient fresh water to produce a 9.5 ppg (1.14 kg/liter) slurry.The 16 inch (406 mm) casing string was cemented with about 2,188 cubicfeet (62 m³) of slurry formulated by mixing about 800 sacks (75,200pounds 34110 kg) of API Class A portland cement with about 1,200 pounds(544 kg) of calcium chloride, about 128(58 kg) pounds of defoamer, about20,000 pounds (9072 kg) of B37/2000 hollow glass microspheres andsufficient fresh water to produce a 9.5 ppg (1.14 kg/liter) slurry. Themicrospheres, defoamer, and calcium chloride were dry blended with thecement and then the dry blended mixture was mixed with the water justprior to cementing the casing strings.

It will be readily apparent to those skilled in the art of cementing oilwells and the like that the utility of the slurry, of the describedmethod, and of the resulting set composition are not dependent on theembodiments shown and that a considerable variation can be permitted, asset out in the scope of the appended claims.

What is claimed is:
 1. In a method of cementing a casing in a wellwherein cement is pumped downwardly through the casing and upwardly intothe annulus between the casing and the wall of the well, wherein theimprovement comprises:pumping a cement slurry which comprises, as itsessential components, hydraulic cement, about 8 to about 50 weightpercent hollow glass microspheres based on the weight of the cement andsufficient water to form a pumpable slurry downwardly through the casingand upwardly into the annulus between the casing and the wall of thewell, wherein the API free water content of said slurry is no more thanabout 2 volume percent based only on said hydraulic cement and saidmicrospheres and said microspheres have ANSI/ASTM D 2840-69 average trueparticle densities of about 0.2 to about 0.5 gm/cm³, ANSI/ASTM D 3102-72hydrostatic collapse strengths of at least 500 psi (3447 kPa) andaverage particle diameters of less than about 500 microns.
 2. In amethod of cementing a casing in a well wherein cement is pumpeddownwardly through the casing and upwardly into the annulus between thecasing and the wall of the well, wherein the improvementcomprises:pumping a cement slurry which comprises hydraulic cement,about 8 to about 50 weight percent hollow glass microspheres based onthe weight of the cement and sufficient water to form a pumpable slurrywith an API free water content of no more than about 2 volume percentdownwardly through the casing and upwardly into the annulus between thecasing and the wall of the well, wherein said slurry is free fromeffective amounts of sodium metasilicate and said microspheres haveANSI/ASTM D 2840-69 average true particle densities of about 0.2 toabout 0.5 gm/cm³, ANSI/ASTM D 3102-72 hydrostatic collapse stengths ofat least 500 psi (3447 kPa) and average particle diameters of less thanabout 500 microns.
 3. In a method of cementing a casing in a wellwherein cement is pumped downwardly through the casing and upwardly intothe annulus between the casing and the wall of the well, wherein theimprovement comprises:pumping downwardly through the casing and upwardlyinto the annulus between the casing and the wall of the well a cementslurry which comprises hydraulic cement, about 8 to about 50 weightpercent hollow glass microspheres based on the weight of said cementand, as substantially all of the water in the slurry, an amount of waterrequired to produce a pumpable slurry with the hydraulic cement andmicrospheres having an API free water content of no more than about 2volume percent, wherein said microspheres have ANSI/ASTM D 2840-69average true particle densities of about 0.2 to about 0.5 gm/cm³,ANSI/ASTM D 3102-72 hydrostatic collapse strengths of at least 500 psi(3447 kPa) and average particle diameters of less than about 500microns.
 4. In a method of cementing a casing in a well wherein cementis pumped downwardly through the casing and upwardly into the annulusbetween the casing and the wall of the well, wherein the improvementcomprises:pumping downwardly through the casing and upwardly into theannulus between the casing and the wall of the well a cement slurrywhich comprises hydraulic cement, about 8 to about 50 weight percenthollow glass microspheres based on the weight of said cement and, assubstantially all of the water in the slurry, an amount of waterrequired to produce a pumpable slurry with the hydraulic cement havingan API free water content of no more than about 2 volume percent and anadditional amount of water equal to about the amount shown in FIG. 4 foreach weight percent of said microspheres, wherein said microspheres haveANSI/ASTM D 2840-69 average true particle densities of about 0.2 toabout 0.5 gm/cm³, ANSI/ASTM D 3102-72 hydrostatic collapse strengths ofat least 500 psi (3447 kPa) and average particle diameters of less thanabout 500 microns.
 5. In a method of cementing a casing in a wellwherein a cement slurry is pumped downwardly through the casing andupwardly into the annulus between the casing and the wall of the well,wherein the improvement comprises:pumping downwardly through the casingand upwardly into the annulus between the casing and the wall of thewell a cement slurry which comprises hydraulic cement, about 8 to about50 weight percent hollow glass microspheres based on the weight of thecement and an amount of water required to produce a pumpable slurry withan API free water content of less than about 2 volume percent, whereinsaid microspheres have ANSI/ASTM D 2840-69 average true particledensities of about 0.2 to about 0.5 gm/cm³, average particle diametersof less than about 500 microns and ANSI/ASTM D 3102-72 hydrostaticcollapse strengths after collapse of about 10 volume percent of saidmicrospheres substantially as shown in FIG.
 2. 6. In a method ofcementing a well, wherein a cement slurry is pumped downwardly throughthe casing and upwardly into the annulus between the casing and the wallof the well, wherein the improvement comprises:pumping downwardlythrough the casing and upwardly into the annulus between the casing andthe wall of the well a cement slurry which comprises hydraulic cement,about 8 to about 50 weight percent hollow glass microspheres based onthe weight of the cement and an amount of water required to produce apumpable slurry with an API free water content of less than about 2volume percent, wherein said microspheres are manufactured by theprocedure described in Beck, et al, U.S. Pat. No. 3,365,315, haveANSI/ASTM D 2840-69 average true particle densities of about 0.2 toabout 0.5 gm/cm³, ANSI/ASTM D 3102-72 hydrostatic collapse strengths ofat least 500 psi (3446 kPa) and average particle diameters of less thanabout 500 microns.
 7. In a method of cementing a well, wherein a cementslurry is placed in the well and maintained therein until the cement hashardened, wherein the improvement comprises:placing in the well andmaintaining therein a cement slurry which comprises, as its essentialcomponents, hydraulic cement, about 8 to about 50 weight percent hollowglass microspheres based on the weight of the cement and sufficientwater to form a pumpable slurry, wherein the API free water content ofsaid slurry is no more than about 2 volume percent based only on saidhydraulic cement and said microspheres and said microspheres haveANSI/ASTM D 2840-69 average true particle densities of about 0.2 toabout 0.5 gm/cm³, ANSI/ASTM D 3102-72 hydrostatic collapse strengths ofat least 500 psi (3447 kPa) and average particle diameters of less thanabout 500 microns.
 8. In a method of cementing a well, wherein a cementslurry is placed in the well and maintained therein until the cement hashardened, which comprises:placing in the well and maintaining therein acement slurry which comprises hydraulic cement, about 8 to about 50weight percent hollow glass microspheres based on the weight of thecement and sufficient water to form a pumpable slurry with an API freewater content of no more than about 2 volume percent, wherein saidslurry is free from effective amounts of sodium metasilicate and saidmicrospheres have ANSI/ASTM D 2840-69 average true particle densities ofabout 0.2 to about 0.5 gm/cm³, ANSI/ASTM D 3102-72 hydrostatic collapsestrengths of at least 500 psi (3447 kPa) and average particle diametersof less than about 500 microns.
 9. In a method of cementing a well,wherein a cement slurry is placed in the well and maintained thereinuntil the cement has hardened, wherein the improvement comprises:placingin the well and maintaining therein a cement slurry which compriseshydraulic cement, about 8 to about 50 weight percent hollow glassmicrospheres based on the weight of said cement and, as substantiallyall of the water in the slurry, an amount of water required to produce apumpable slurry with the hydraulic cement and microspheres having an APIfree water content of no more than about 2 volume percent, wherein saidmicrospheres have ANSI/ASTM D 2840-69 average true particle densities ofabout 0.2 to 0.5 gm/cm³, ANSI/ASTM D 3102-72 hydrostatic collapsestrengths of at least 500 psi (3447 kPa) and average particle diametersof about less than 500 microns.
 10. In a method of cementing a well,wherein a cement slurry is placed in the well and maintained thereinuntil the cement has hardened, wherein the improvement comprises:placingin the well and maintaining therein a cement slurry which compriseshydraulic cement, about 8 to about 50 weight percent hollow glassmicrospheres based on the weight of said cement and, as substantiallyall of the water in the slurry, an amount of water required to produce apumpable slurry with the hydraulic cement having an API free watercontent of no more than about 2 volume percent and an additional amountof water equal to about the amount shown in FIG. 4 for each weightpercent of said microspheres, wherein said microspheres have ANSI/ASTM D2840-69 average true particle densities of about 0.2 to 0.5 gm/cm³,ANSI/ASTM D 3102-72 hydrostatic collapse strengths of at least 500 psi(3447 kPa) and average particle diameters of about less than 500microns.
 11. In a method of cementing a well wherein a cement slurry isplaced in the well and maintained therein until the cement has hardened,wherein the improvement comprises:placing in the well and maintainingtherein a cement slurry which comprises hydraulic cement, about 8 toabout 50 weight percent hollow glass microspheres based on the weight ofthe cement and an amount of water required to produce a pumpable slurrywith an API free water content of less than about 2 volume percent,wherein said microspheres have ANSI/ASTM D 2840-69 average true particledensities of about 0.2 to about 0.5 gm/cm³, average particle diametersof less than about 500 microns and ANSI/ASTM D 3102-72 hydrostaticcollapse strengths after collapse of about 10 volume percent of saidmicrospheres substantially as shown in FIG.
 2. 12. The method of claims1, 2, 3, 4, 5, 7, 8, 9, 10 or wherein said microspheres are manufacturedby the procedure described in Beck, et al, U.S. Pat. No. 3,365,315. 13.In a method of cementing a well, wherein a cement slurry is placed inthe well and maintained herein until the cement has hardened, whereinthe improvement comprises:placing in the well and maintaining herein acement slurry which comprises hydraulic cement, about 8 to about 50weight percent hollow glass microspheres based on the weight of thecement and an amount of water required to produce a pumpable slurry withan API free water content of less than about 2 volume percent, whereinsaid microspheres are manufactured by the procedure described in Beck,et al, U.S. Pat. No. 3,365,315, have ANSI/ASTM D 2840-69 average trueparticle densities of about 0.2 to about 0.5 gm/cm³, ANSI/ASTM D 3102-72hydrostatic collapse strengths of at least 500 psi (3447 kPa) andaverage particle diameters of less than about 500 microns.
 14. Themethods of claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 13 whichcomprises sufficient water to form a pumpable slurry with at least anAPI minimum water content and an API free water content of no more thanabout 2 volume percent.
 15. The method of claims 1, 2, 3, 4, 5, 6, 7, 8,9, 10, 11, or 13 which comprises sufficient water to form a pumpableslurry with at least an API minimum water content and an API free watercontent of less than about 1 volume percent.
 16. The method of claims 1,2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 13 which comprises sufficient water toform a pumpable slurry with an API normal water content.
 17. The methodof claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 13 wherein saidmicrospheres have ANSI/ASTM average true particle densities of about 0.3to about 0.4 gm/cm³.
 18. The method of claims 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 11 or 13 wherein the average particle diameters of said microspheresare about 10 to about 300 microns.
 19. The method of claims 1, 2, 3, 4,5, 6, 7, 8, 9, 10, 11 or 13 wherein said hydraulic cement is portlandcement.
 20. The method of claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 13wherein said slurry comprises about 10 to about 30 weight percent hollowglass microspheres based on the weight of the cement.
 21. The method ofclaims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 13 wherein said slurrycomprises, as substantially all of the water in the slurry, an amount ofwater required to produce a slurry with the hydraulic cement having atleast on API minimum water content and an API free water content of nomore than about 2 volume percent and an additional amount of water equalto about 1.2 to about 2.2 weight percent water based on the weight ofthe cement for each weight percent of said microsphere.
 22. The methodof claim 20 wherein said slurry comprises, as substantially all of thewater in the slurry, an amount of water required to produce a slurrywith the hydraulic cement having at least an API minimum water contentand an API free water content of no more than about 2 volume percent andan additional amount of water equal to about 1.3 to about 1.8 weightpercent water based on the weight of the cement for each weight percentof said microsphere.
 23. The method of claims 1, 2, 3, 4, 5, 7, 8, 9, 10or 13 wherein said hollow glass microspheres are further characterizedin that said microspheres having ANSI/ASTM average true particledensities of about 0.3 gm/cm³ have ANSI/ASTM hydrostatic collapsestrengths after collapse of about 10 volume percent of said microspheresof at least about 1000 psi (6895 kPa).
 24. The method of claim 23wherein said hollow glass microspheres are further characterized in thatsaid microspheres having ANSI/ASTM average true particle densities ofabout 0.4 gm/cm³ have ANSI/ASTM hydrostatic collapse strengths aftercollapse of about 10 volume percent of said microspheres of at leastabout 4000 psi (27,579 kPa).
 25. The method of claims 1, 2, 3, 4, 6, 7,8, 9, 10 or 13 wherein said microspheres are further characterized inthat said microspheres have hydrostatic collapse strengths aftercollapse of about 10 volume percent of said microspheres substantiallyas shown in FIG.
 2. 26. A method for insulating and lining the exteriorof a well casing while decreasing likelihood of hydraulically fracturingsaid well, comprising:preparing a pumpable lightweight hydraulic cementslurry comprising a mixture of a finely-ground hydraulic cement; betweenabout 8 and 50 weight % (by weight of cement) of finely divided hollowglass microspheres having a density nonsubstantially exceeding anANSI/ASTM average true particle density of 0.5 gm/cm³, and ANSI/ASTMhydrostatic collapsed strength of at least 500 psi (3447 kPa), and asoftening temperature not less than about 480° C., said microsphereshaving an average particle diameter in the range of about 10 to about300 microns; and water in an amount to produce a maximum slurryconsistency not substantially exceeding 30 B c (API) and less than thatproducing 2% to volume percent free water content (API), pumping saidslurry downwardly through said casing and upwardly between said casingand walls of said well, and solidifying said slurry between said casingand said well to produce set cement there between, having a long-termcompressive strength at well temperatures in excess of about 230° F.(110° C.) not substantially lower than the initial compressive strengthof said set cement.
 27. A method in accordance with claim 26 in whichsaid cement in said casing is drilled out after placement, commencingnot substantially in excess of 1 day,the specific gravity of said cementslurry is in the range of about 1.1 to about 1.4 and the 24-hourcompressive strength of said set cement is at least about 250 psi (1724kPa).